1. FIELD OF THE INVENTION
The present invention relates to liquid phase, acid catalyzed alkylation of isoparaffin hydrocarbon with olefin hydrocarbon for production of alkylate having high octane value and being suitable for gasoline blending stock. More particularly, the present invention relates to such an alkylation process wherein improved acid catalyst comprising a selected ratio of trifluoromethane sulfonic acid with sulfuric acid is employed.
2. DESCRIPTION OF THE PRIOR ART
Liquid phase alkylation processes wherein isoparaffin hydrocarbons, such as isobutane, etc. are alkylated with olefin hydrocarbons such as propylene, butylenes, etc. for production of alkylate products comprising highly branched C.sub.7 -C.sub.8 range paraffin hydrocarbons having high octane values, are well known and widely practiced. In such alkylation processes, reactant hydrocarbons are generally contacted in the liquid phase, at temperatures in the range of from about 0.degree.-100.degree. F., in the presence of acid alkylation catalysts under conditions of good mixing to produce the desired alkylate hydrocarbons. Reaction pressures, which may vary from about ambient to superatmospheric, are sufficient to maintain reactants in the liquid phase. Higher pressures than that required to maintain reactants in the liquid phase apparently have no substantial effect upon such alkylation reactions. Acid catalysts may be selected from a wide range of strong acids including sulfuric acid and fluorosulfonic acid. Generally, liquid alkylation catalysts such as sulfuric acid, trifluoromethane sulfonic acid and fluorosulfonic acid are substantially immiscible with isoparaffin hydrocarbon reactants. Consequently, reactant hydrocarbons and liquid alkylation catalysts are contacted in an alkylation zone under conditions of high shear mixing sufficient to form an emulsion of immiscible hydrocarbon and catalyst. Detailed description of such alkylation processes is not required herein, as such processes are well known and widely practiced in the prior art.
In such alkylation processes side reactions take place in addition to desired alkylation reactions wherein 1:1 olefin-isoparaffin adducts are produced. For example, olefin hydrocarbons tend to polymerize in the presence of strong acid catalysts to form C.sub.12 and higher molecular weight polymers; also such polymers tend to crack in the presence of the strong acid catalyst forming low octane C.sub.5 -C.sub.7 hydrocarbons. These side reaction products, e.g., olefin polymers and cracked products, have relatively low octane values and are not as desirable as the 1:1 olefin isoparaffin alkylation products for use as gasoline blending stocks.
In order to maximize the yield of the desired alkylate products at the expense of the undesired side reaction products, care is taken to insure good contact of isoparaffin reactants with olefin reactants in the presence of alkylation catalysts under reaction conditions which favor formation of alkylate hydrocarbon products over side reaction products. For example, reaction temperatures are maintained in the range of about 0.degree. F. to about 100.degree. F. wherein the alkylation of isoparaffins with olefins proceeds at economically attractive rates and wherein olefin polymerization rates are not so great as to consume excessive amounts of olefin reactant. Further, a substantially stoichiometric excess of isoparaffins over that required to react with olefin is employed to insure contact of isoparaffins with olefins under alkylation reaction conditions. Further, as olefin reactants are generally substantially more soluble in acid catalysts than are isoparaffin reactants, high-shear mixing of the alkylation reaction mixture is generally provided sufficient to form emulsions of acid catalysts and hydrocarbon reactants. Formation of such emulsions increases the contact area between acid catalyst, containing dissolved olefins with isoparaffin hydrocarbons, thus materially improving yield and quality of alkylate product.
Commonly, where sulfuric acid is the selected acid catalyst, the alkylation reaction emulsion is one wherein isoparaffin hydrocarbon is emulsified in a continuous acid phase, although it is known to employ reaction emulsions comprising acid catalyst phase suspended in a continuous isoparaffin hydrocarbon phase. Alkylation reaction mixtures comprising about 40-70 volume percent sulfuric acid catalyst and about 60-30 volume percent hydrocarbon phase are known to form emulsions wherein the acid phase is continuous.
It is well-known that sulfuric acid is a particularly effective catalyst for the alkylation of isoparaffin hydrocarbon with olefin hydrocarbon. Additionally, it is known that sulfonic acids, including fluorosulfonic (HFSO.sub.3), trifluoromethane sulfonic (CF.sub.3 SO.sub.3 H) chlorosulfonic, and various organic sulfonic acids are effective alkylation catalysts used alone or in admixture with other strong acids. For example see: U.S. Pat. No. 3,708,533, Olah, which teaches alkylation of paraffins with olefins in a reaction catalyzed by a mixture of Lewis acid (e.g. SbF.sub.5) and a Bronsted acid (e.g. CF.sub.3 SO.sub.3 H); U.S. Pat. No. 2,313,013 which teaches alkylation of an isoparaffin with an olefin hydrocarbon employing fluorosulfonic acid as catalyst is superior to alkylation reactions employing H.sub.2 SO.sub.4 catalyst; U.S. Pat. No. 3,231,633 wherein it is taught that the catalytic activity of fluorosulfonic acid is improved by addition of a minor amount of H.sub.2 SO.sub.4 (e.g. not less than 50 weight percent HFSO.sub.3 is the catalyst mixture); U.S. Pat. No. 2,259,723, Ballard et al, which teaches the use of chlorosulfonic and fluorosulfonic acids in admixture with hydrogen halides; U.S. Pat. No. 3,766,293, Parker et al which teaches the use of "fluorosulfuric" acid in combination with a minor amount of catalyst promoter which may include sulfuric acid; U.S. Pat. No. 3,231,633 Kramer, which teaches an alkylation process employing fluorosulfonic acid in combination with up to about 50 weight percent sulfuric acid; and U.S. Pat. No. 2,425,572, Slotterbeck, which teaches an alkylation process wherein a minor portion (1-10%) of chlorosulfonic acid or other substituted sulfonic acids, such as aliphatic sulfonic acids, are added to the sulfuric acid alkylation catalyst.
Trifluoromethane sulfonic acid, as well as other sulfonic acids, is relatively expensive compared to sulfuric acid. Thus, one of the major disadvantages of using trifluoromethane sulfonic acid, or other sulfonic acids, as alkylation catalyst is high cost of these acids. In alkylation reactions, acid catalyst is consumed at rates ranging from about 0.1 up to about 1.0 or more pounds of catalyst per gallon of alkylate product as a result of side reactions such as hydrolysis and reactions with olefin hydrocarbons. Consequently, the expense of providing sulfonic acids as the major components of alkylation catalysts is generally prohibitively expensive in commercial alkylation processes.